The present invention relates to a hot-wire type air flow meter based on the use of an auxiliary flow passage and, more particularly, to an engine control device which is suitable for use as an intake air flow-rate sensor for a gasoline engine of an automobile.
In various internal combustion engines, particularly in a gasoline engine of an automobile, the range of control thereof is very wide in respect of the number of revolutions and output and the engine is subjected to strict regulation of exhaust gas. Therefore, an accurate air fuel ratio control is always required to be performed irrespective of the operational conditions of the engine.
Under the above-mentioned existing circumstances, a microcomputer-control type engine control device has in recent years been widely adopted in which the air fuel ratio or the ignition timing is controlled using a microcomputer operating on the basis of a synthetic decision on various operational conditions of the engine including an intake air flow thereof.
Meanwhile, what is called "hot-wire type air flow meter" has recently been employed as an air flow sensor for such engine control. Examples thereof are disclosed in Japanese Patent Unexamined Publications Nos. 58-109815 and 58-109816. In these prior art examples, an intake air passage of an internal combustion engine is constituted by a primary flow passage and auxiliary flow passage allowed to project within the primary flow passage and having a specified length, the auxiliary flow passage having a portion extending substantially in parallel with the flow of the air passing through the primary flow passage. Namely, said prior art examples show the hot-wire type air flow meter of auxiliary flow passage system, in which a sensor element for an intake air flowrate is disposed in said auxiliary flow passage.
An outlet of the auxiliary flow passage is opened in the vicinity of an enlarged section of the primary flow passage, or is opened at a straight pipe section.
However, in the above-described prior art, no consideration is given to the stability of air flow at the outlet of the auxiliary flow passage where the sensor element is disposed. Namely, the turbulence of air flow, which has nothing to do with the pulsation of air flow due to the revolutions of the engine involved, vigorously occurs at said outlet, thus raising a problem that such turbulence of air flow is likely to have undesirable effects upon the metering of the intake air flow rate.
To cope with this problem, a means for solving the same has been proposed in which a rectifying member such as a mesh member or honeycomb member is provided over the intake air flow passage so as to stabilize the flow of intake air. In this proposal, however, a considerable increase in manufacturing cost, inherently results.